Speed control for electric motors



Oct. 16, 1951 J JONES r AL 2,571,454

SPEED CONTROL FOR ELECTRIC MOTORS Filed July 2, 1948 6 at ca 4o .20

Patented Oct. 16, 1951 SPEED CONTROL FOR ELECTRIC MOTORS Jesse E. Jones, Wauwatosa, and James B. Reeves, I Whitefish Bay, Wi.s., assignors to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application July 2, 1948, Serial No. 36,718

This invention relates to speed control means for electric motors.

While not limited thereto the present invention is particularly applicable for governing the operation of motor-driven winders such as are used in the packaging of textile thread.

A primary object of the invention is to provide improved control of a simple, sensitive and reliable character which will provide for reeling of lengths of material at substantially constant tension.

Another object is to accomplish such control by electronic means under the direction of a transformer which produces a control signal varying in magnitude in response to variation in the length of a loop formed by the material to be tensioned, and

Another object is to provide a transformer affording a strong control signal variable through a wide range and to obtain such a signal through the medium of means which is substantially frictionless and operable without producing any retarding torque, whereby said means will respond to a very small operating force such as may be transmitted through textile thread without undue strain thereon.

Other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate certain preferred embodiments of the invention which will now be described. it being understood that the embodiments illustrated are susceptible of various modifications without departing from the scope of the appended claims.

In the drawings:

Figure 1, which is in part diagrammatic and in part schematic, illustrates a control system for motor-driven thread winding apparatus incorpo rating the invention. a

Fig. 2 is a view in front elevation of a control device shown schematically in Fig. 1.

Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2.

Referring to Fig. 1, it shows a motor-driven thread winder comprising a reel or bobbin It on which a length ofthread II is to be wound by means of a three phase alternating current motor I! which drives bobbin l through a shaft l3. The thread II is shown as being supplied from a spool ll which may here be assumed to be driven at a constant rotational speed through means not shown.

Idler rollers I5 and it are suitably placed in the path of thread I l between spool It and bobbin "to facilitate the formation of a loop or festoon 4 Claims. (Cl. 318-6) in thread II which passes through a thread guide or eyelet l'l secured to a tension arm l8 of a control devic I9. Tension arm I8 is non-rotatably secured to a shaft of control device l9 and is provided with a weight 2| which may be variously positioned thereon to adjust the force exerted through the eyelet H on thread II.

The motor 12 is shown as being of the squirrel cage type, its primary being provided with supply terminals T T and T which are respectively connected to supply lines L L and L of a three phase alternating current source; terminal T being connected to supply line L in series with a resistor 22. Motor l2 may be any suitable three phase alternating current motor having high secondary resistance, a motor of the so-called "torque" type being preferred.

A gas filled trioue tune 23 is connected at its anode 23 to terminal T of motor I: and at its catnoue 23 to termlnal T of said motor. Tube 23 is provided with a control grid 23 which is connected to one end of a coil 24 of control device \3 through a resistor 25 and a rectiner 26. A capacitor 2| and a resistor 28 are connected in parallel between the cathode 23 of tube 23 and the interconnection between the resistor 25 and rectifier 26, whereas a point common to cathode 23 resistor 28 and capacitor U is connected to the end of coil 24 opposite the end aforementioned. A second capacitor 29 connected between cathode 23 and control grid 23 of tube 23 is provided for bypassing transient and harmonic currents.

Control device is is also provided with a coil 30 which is connected across supply lines L and L and which has its longitudinal axis curved concentrically with shaft 20. An arm 3| is nonrotatably secured to shaft 20 and is adapted to assume various angular positions in response to rotational movement of shaft 20 to vary the magnetic flux linking the coils 24 and 30. A frame 32 (shown in broken lines) is provided to support the aforedescribed components of control device [9. The arm 3i and frame 32 are preferably formed of magnetic material such as soft steel to provide a low reluctance path for the magnetic flux induced by energization of coil 30. A preferred form for control device l9 will be hereinafter described in detail.

As will be apparent, the coil 30 will be energized whenever supply line L and L ar energized and as a result a voltage will be induced in the coil 24. In the position of arm 3| depicted in Fig. l,

the maximum possible magnetic coupling ob- 7 age induced in coil 24 will be reduced in proportion to the decrease in magnetic coupling. Movement of arm 3| upwardly beyond a certain point will not efiect any appreciable reduction in the .magnetic coupling, such point corresponding to minimum voltage induced in coil 24. The action of control device l9 to produce a voltage varying in value between a maximum and minimum in relation to mechanical movement of a part thereof is utilized to control the conduction of tube 23 for regulation of the speed of motor 12 as will now be explained.

Tube 23 is only capable of conduction when the potential polarities of its anode 23 and cathode 23 are respectively positive and negative which occurs onalternate half cycles when the voltage at motor terminal T is positive with respect to the voltage at terminal T of said motor. As a further condition to conduction of tube 23 the potential of its control grid 23 must rise to a critical value, at which conduction is initiated during such half cycles. As will now be explained the potential attained by control grid 23 during each such half cycle is dependent upon the peak value of the induced voltage in coil 24 attained during preceding half cycle when tube 23 is nonconductive.

The coil 24 is so connected that on the half cycles when tube 23 is non-conductive, the voltage induced in coil 24 will effect current flow through rectifier 26 and will charge capacitor 21 to a potential corresponding to the peak value of voltage induced in coil 24. As a result the potential of control grid 23 will be made negative with respect to the potential of cathode 23 in an amount corresponding to the potential to which capacitor 21 is charged and in any event to an amount blocking conduction by tube 23. Unblocking of the tube is then dependent upon discharge of the capacitor through resistor 28, which discharge begins as soon as the surge of current through the coil 24 reaches and passes its peak. According to the charge of the capacitor and its rate of discharge unblocking of tube 23 is effected during the next half cycle or may be continued throughout the next half cycle. In the latter event ,the tube is rendered idle while in the former event the tube conducts for part or all of the half cycle depending upon the move ment at which the negative voltage impressed upon the control grid 23 is reduced sumciently by discharge of the capacitor to permit the tube to become conductive with its anode and cathode respectively subjected to positive and negative voltages. When a condition of maximum magnetic coupling obtains in control device I9, capacitor 21 will be charged to such a potential that its subsequent discharge will consume the next half cycle to thus rendering the tube idle during such half cycle. Reducing the degree of magnetic coupling in control device 59 will of course reduce the potential to which capacitor 21 is charged, and if it be reduced sufiiciently, then the negative potential impressed upon the control grid 23 will be reduced to permit conduction by tube 23 during at least part of the half cycle following the half cycle during which the capacitor reserves its charge. A further reduction of magnetic coupling in control device I9 will result in initiation of conduction of tube 23 earlier in its conducting half cycles.

Conduction of tube 23 effects a single phase connection of motor l2, both its terminals T and '1 beingconnected to supply line L. Consequently while tube 23 conducts, the motor primary isaflorded unbalanced excitation. The resistor 22 normally effects a slight voltage unbalance of the motor primary but the resistor need not be of a value such as to efiect more than a negligible unbalance. Assuming that tube 23 conducts during portions of alternate half cycles the motor periodically changes from a condition of almost balanced to unbalanced excitation which causes the speed of the motor to decrease. The greater the average current conducted by tube 23, the

greater will be the reduction in speed of motor [2.

As will be understood, tension arm l8 acting through the eyelet l1, maintains the thread It under constant tension in its passage from spool l4 to the bobbin Ill so long as a loop is maintained between rollers l5 and IS; the amount of tension being determined by the magnitude and position of weight 2| on arm l8. The diameter of bobbin II) will progressively increase due to successive winding layers of thread II and the peripheral speed of bobbin ID will tend to increase with consequent tendency to wind thread faster than it is fed from spool l4. This causes the aforedescribed loop in thread I l to progressively decrease in length as the bobbin III increases in diameter. As a result tension arm I is progressively moved in the clockwise direction with consequent progressive reduction in the magnetic coupling between coils 24 and 30 of control device I9. Thus it will be seen that the speed of motor I2 will be progressively reduced so that the peripheral speed of bobbin III will be maintained substantially constant relative to the peripheral speed of spool l4 which decreases as thread is unwound therefrom.

Whereas it has here been assumed that thread H is unwound from a spool driven at a constant rotational speed such a condition of thread supply is not essential to proper functioning of the control system. It is only necessary that there be some continual resistance at the supply end to thread advance.

As will be understood by those skilled in the art, thread being wound will be subject to certain transient or cyclic variations which tend to vary its tension and cause hunting action of tension arm IS. The eilect of such hunting action on control device l9 can be best reduced by increasing the effective length of tension arm l8.

Referring to Figs. 2 and 3, they show a preferred form for control device H! which will now be described in detail.

The coils 24 and 30 are provided with individual carriers 24 and 30 of a spool type, which are preferably formed of a non-magnetic material such as a linen based Bakelite. Coil 30 is rigidly secured to plate 32 by a. plurality of attaching lugs 32 which are sheared from plate 32, peripherally about an end flange of carrier 30, and bent over said end flange. An opening extending axially through carrier 30 is provided to freely accommodate the shaft 20 which preferably is steel and on which the coupling arm 3| is nonrotatably secured. Coil 24 is secured to plate 32 in a spaced relation to coil 30 by means of a screw 33 which extends through a central opening in carrier 24 and which takes into a threaded opening formed in plate 32.

An inverted U-shaped bearing bracket 34 formed of a non-magnetic material such as brass is secured to plate 32 in a straddling relation to coil 30 by means of screws 35 which extend through openings in oppositely depending end portions of said bracket and take into threaded openings in plate 32. The portion of bracket 34 overlying coil 30 is provided with a threaded opening concentric with respect to shaft 20 to accommodate a screw type bearing seat 36 for shaft 2|). The end of seat 36 adjacent shaft 20 i provided with an inverted conical recess to partially accommodate a small polished steel ball 31 which is also partially accommodated in a corresponding recess formed-in the adjacent end of shaft 20. Seat 36 is slotted at its other end to facilitate its adjustment relative to bracket 34 so that its recessed end may be moved toward or away from shaft 23. A desired adjustment of seat 36 is maintained by means of a nut 36" which has engagement with the threaded shank of seat 33 to be clamped a ainst bracket 34 to lock seat 36 in adjusted position.

Shaft 20, as aforedescribed, extends through the coil 30 and also extends through and beyond plate 32, the latter being provided with a clearance opening to accommodate said shaft. A ball bearing 38 is fitted on shaft 20 with a driving fit to afford radial bearing support for the latter within a bearing housing 33 which is secured to plate 32 oppositely of the coil 30. The housing 33 is secured to plate 32 by a plurality of screws 43 which pass through said housing marginally thereof and take into threaded openings formed in plate 32. Housing 39 is suitably formed interiorly to tightly engage with ball bearing 33 peripherally about the outer race of the latter and to restrain the same and shaft against any substantial axial movement. A clearance opening extending through the end wall of housing 33 is provided to accommodate shaft 20 which extends beyond said housing.

The coupling arm 3| is provided with a hub portion 3|" which has an opening to accommodate with clearance a reduced threaded end portion 20 of shaft 20. Arm 3| is non-rotatably secured to shaft 20 by a nut 4| threaded onto the portion 20" of shaft 20 to clamp said arm against an annular shoulder 20 formed on shaft 23. Arm 3| is provided with a straight portion 3| which is integrally connected t the hub 3| at one end thereof and which merges at its other end with a portion 3| which depends perpendicularly toward the plate 32. The portion 3| is so proportioned in length as to freely clear the carrier 24' upon angular movement of arm 3|. Arm 3| is also provided with a circular segment portion 3| which is integrally connected with the hub portion 3|; said portion 3| affording limitation of angular movement of arm 3| by engagement of its radial sides with the portions of bracket 34 which extend perpendicularly with respect to plate 32. Preferably, the portion 3| is so proportioned as to afford balance of the arm 3| with respect to its pivotal axis.

Tension arm I8 is non-rotatably secured, in any preferred manner, to the portion of shaft 23 which extends beyond the housin 33. While tension arm I8 is depicted in Figs. 2 and 3 as being mounted on shaft 20 in certain relation with respect to the portion 3| of coupling arm 3|, it may be mounted in various relatiom to coupling arm 3| according to the particular application of control device l3. Tension arm I8 is depicted as having the open eyelet II as pre- 6 ferred thread engaging means, but as will be apparent this may be replaced, if desired, by a roller when used in connection with web materials or the like.

As will be apparent when coupling arm 3' is in the position depicted in full lines in Fig. 2, the condition of maximum magnetic coupling between coils 24 and 30 is afforded and the voltage induced in coil 24 will be at a maximum. Conversely when coupling arm 3| is in the position depicted in broken lines in Fig. 2, a condition of minimum magnetic coupling between coils 24 and 30 is afforded and consequently the voltage induced in coil 24 will be at a minimum. Accordingly, the voltage induced in coil 24 can be varied between minimum and maximum values by variously positioning coupling arm 3| between the positions aforedescribed.

As will be understood by those skilled in the art coil 24 will have a considerably greater number of coil turns than coil 30 and the ratio of coil turns will depend upon the particular application and the circuit to which the coil 24 is connected.

What we claim as new and desire to secure by Letters Patent is:

1. In combination, a polyphase induction motor for imparting movement to material having a loop path, the primary of said motor having at least three terminals for connection to three lines of a supply circuit, an electron tube connected between two of said motor terminals and serving while conducting to effect voltage unbalance of the motor primary for motor speed reduction, said tube having a control electrode, and means controlling said tube through the medium of said electrode to render conduction by said tube a function of variation in length of the loop of the looped material, said means comprising a transformer adjustable in respect of magnetic coupling of its coils by. variation in length of the loop and serving by consequent variation in its output voltage to regulate the potential impressed upon said electrode.

2. In combination, a polyphase induction motor having at least three primary terminals for connection to three lines of a supply circuit, an electron tube connected between two of said primary terminals and serving while conducting to effect voltage unbalance of the motor primary for speed reduction, said tube having a control electrode, and means controlling said tube for varying conduction thereof by varying the voltage impressed on its control electrode, said means comprising a transformer adjustable in respect of magnetic coupling of its coils and serving by consequent variation in its output voltage to regulate the potential impressed upon said electrode.

3. In combination, a polyphase induction motor for imparting movement to material having a loop path, the primary of said motor having at least three terminals for connection to three lines of a supply circuit, a gaseous electron tube connected between two of said motor terminals and serving while conducting to effect voltage unbalance of the motor primary for motor speed reduction, said tube having a control electrode, and means controlling said tube through the medium of said electrode to render conduction by said tube a function of variation in length of the loop of the looped material, said means comprising a transformer adjustable in respect of magnetic coupling of its coils by variation in length of the loop and serving by consequent variation in its output voltage to regulate the potential impressed upon said electrode, for blocking discharge by said tube or for permitting in alternate haii cycles conduction by said tube for varying parts of such half cycles.

4. In combination, a polyphase induction motor having for its primary at least three terminals for connection to three lines of a supply circult, a gaseous electron tube connected between two of said motor terminals and serving while conducting to eifect voltage unbalance of the motor primary for motor speed reduction, said tube being capable of conduction only during a1- ternate halt cycles and having a control grid to regulate its action in such half cycles, and control means for said grid comprising a capacitor and a transformer, said transformer being adjustable in respect of magnetic coupling of its coils and serving during the half cycles between those aforementioned to impress upon said capacitor a potential varying with the adjustment of said transformer and said capacitor delaying attainment by said electrode of a critical value according to the charge of said capacitor.

JESSE EKJONES. JAMES B. REEVES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,068,116 Shatter Jan. 19, 1937 2,146,869 White Feb. 14, 1939 2,275,192 Bailey Mar. 3. 1942 2,351,759 Grundmann June 20, 1944 FOREIGN PATENTS Number Country Date 335,309 Germany Mar. 30, 1921 879,311 France Feb. 19, 1943 

